Latex coatings offer the advantages of low solvent content, toughness, high molecular weight polymers, fast drying times, and easy clean-up. However, they lack the convenient autoxidative cross-linking mechanism associated with drying oils and their derived alkyd resins.
Alkyd resins are made by condensation polymerization of dihydric or polyhydric alcohols, polybasic acids and a drying oil glyceride or acid. Typically, the drying-oil glyceride form is converted to a monoglyceride, which is then reacted with an acid or acid anhydride to create an alkyd resin. When such resins are applied as coatings, the polymer backbones cross-link through polymerization of the drying-oil upon exposure to oxygen. The presence of trace amounts of certain metal salts, such as cobalt naphtheneate, serve as catalysts to attain sufficiently rapid cure times. Very hard durable coatings are formed from alkyd resin paints.
The traditional alkyd resin coating systems are a solvent based, low molecular weight materials that offer good penetration into wood and other porous substrates. However, their use has become increasingly undesirable on account of their high volatile solvent content (VOC). Traditional alkyd resin coating systems are also undesirable on account of their slow drying times.
In order to provide a more environmentally acceptable system, considerable effort has been devoted to modifying alkyd systems so they can be water dispersible. One approach receiving attention is a hybrid system wherein an acrylic resin is formed by emulsion polymerization in the presence of a previously dispersed alkyd resin in an attempt to combine the best features of both. Although potential benefits can be seen in some cases, the hybrid systems generally suffer from a mutual incompatibility of the components that can be problematic. As alkyd content increases, phase separation and haze develop in unpigmented films. See T. Nabuurs et al., Prog. In Org. Coatings, 27, 163 (1996).
A recently introduced method of pre-emulsion or mini-emulsion polymerization (see Schork et al., J. Appl. Polym. Sci., 60, 2069 (1996) and E. M. S. van Hamersveld et al., FATIPEC congr., 24, VOL D/247 (1998)) differs from conventional emulsion polymerization by proceeding in a single stage while not requiring diffusion of the vinylic monomers through an aqueous phase. During mini-emulsion polymerization, the monomers are pre-emulsified as small uniform particles along with a purposely added hydrophobic component. The hydrophobic component retains the more water-soluble monomers within the original particles which are the locus of the polymerization. Hence, there is no second generation of particles as there is in conventional emulsion polymerization. Thus, mini-emulsion polymerization provides a more efficient inclusion of highly water-insoluble species such as an alkyd resin. Schork et al. use a polymeric hydrophobe as an additive to a solution of the alkyd resin in vinylic monomers which are then dispersed in an aqueous solution of surfactants. Van Hamersveld et al. used essentially the same procedure, but with hexadecane as the hydrophobic additive. The dispersions in both cases were then homogenized with ultra sound or high pressure impingement.
The polymerizations of Schork et al. were initiated with persulfate at 60.degree. C. or greater while those of van Hamersveld et al. were initiated with a redox system at 30.degree. C. Schork et al. demonstrated that their mini-emulsion method gave grafting of acrylic monomers to co-emulsified alkyd resins due to the participation of 20 to 30% of the double bonds in the drying oil structures from the alkyd in the polymerization. Van Hamersveld et al. also reported grafting. Both Schork et al. and van Hamersveld et al. claimed more homogenous blends of the mutually incompatible acrylic and alkyd resins, and claimed that the resulting latices formed films that underwent autoxidative cure. However, Schork et al. reported that haze and non-uniformities developed in the films as the amount of alkyd resin was increased.
A more direct approach to introducing autoxidative cure to a latex system was disclosed by the inventor of the present invention, John C. Saam, in U.S. Pat. No. 5,750,751. This patent describes copolymerization of vinylic monomers with glycol co-esters of drying oil acids and .alpha.,.beta.-unsaturated polymerizable carboxylic acids using aqueous emulsion polymerization. The glycol co-esters of drying oil acids and .alpha.,.beta.-unsaturated polymerizable carboxylic acids, which have both an acrylate functional group and drying oil functionalities, were significantly more reactive than the alkyd resins of either Schork et al. or van Hamersveld et al. Also, there was no dilution of the drying oil function by other ingredients which would contribute to incompatibility, such as the polyester segments which are present in alkyd resins. Accordingly, the co-polymers described in U.S. Pat. No. 5,750,751 did not exhibit any incompatibility problems, formed clear continuous films, and underwent autoxidative cure in the presence of a drying catalysts. However, in order to obtain uniformly clear continuous films, the illustrated coating composition in U.S. Pat. No. 5,750,751 contained 9 weight percent on total emulsion or 18 weight percent on solids of a volatile coalescing solvent, namely 2-butoxyethanol. Volatile coalescing solvents are commonly used in latex formulations to facilitate uniform film formation by acting as transient plasticizers, and by reducing the glass transition temperature and initial hardness of freshly deposited films. See J. B. Clarke et al., Waterborne Coatings and Additives, D. R. Kasa and W. D. Davies editors, The Royal Society of Chemistry, Publ. No. 195, 1995, page 18.
However, the use of coalescing solvents is unacceptable when more environmentally benign coating systems are desired. It has been determined that the coating compositions illustrated in U.S. Pat. No. 5,750,751 must contain a coalescing solvent to obtain a uniform clear continuous film when emulsion copolymerizations are conducted at temperatures exceeding 60.degree. C. The high gel content is probably due to side reactions which lead to grafting and copolymerization through the double bonds of the drying oil structures which are pendant from the copolymer chain.